1. Field
The disclosure relates to the field of the manufacture of tires and more particularly to the vulcanizing of very wide tires using molds referred to as self-locking molds.
2. Description of Related Art
Traditional molds are made up of several separate components which, when brought relatively closer together, delimit a substantially toroidal molding space. They essentially comprise two shells for molding the side walls of the tire, and several peripheral segments, situated axially between the shells, for molding the tread. All of these components are brought closer together by a mechanism external to the mold.
The green tire for vulcanizing is introduced into the mold and a curing membrane presses the tire firmly against the mold so as to imprint the pattern of the tread and obtain a vulcanized tire of precise dimensions. The pressure applied by the tire to the mold components is reacted by mechanisms which transfer the forces to the chassis of the curing press in order to keep the mold closed throughout the curing time.
More specifically, the invention is concerned with molds referred to as self-locking molds which have the special feature of comprising shells and segments which collaborate to withstand the pressure so that the components of the mold remain in the closed position without the aid of additional external wedging or maintaining components.
Publication EP 0 436 495, or alternatively publication EP 1 918 087, describe a mold of this type in which each segment comprises at its lateral edges a projection extending radially towards the inside of the mold and equipped with a lip extending axially towards the inside of the mold. The radially outer end of each shell terminates in a complementary volume comprising a projection equipped with a lip. The said lips collaborate to keep the mold closed when this mold is subjected to an internal pressure that has a tendency to part the shells axially from one another and cause the sectors to retreat radially.
The mold comprises frustoconical bearing surfaces on the lips of the said segments and of the said shells. The said frustoconical surfaces are inclined by an angle α with respect to the axis of the mold, so that the extensions of the line of these frustoconical surfaces on a meridian plane passing through the axis of the mold intersect on the radially internal side of the segment concerned.
In order for the mold always to remain closed, the angle α is calculated so that the effect of the internal pressure on the shells which has a tendency to cause the segments to move closer towards the centre of the mold is dominant over the effect of the internal pressure on the segments that have a tendency to cause the segments to part radially and, by transmission of the radial forces via the frustoconical surfaces, to cause the shells to move axially closer together.
In this respect, the higher the internal pressure, the greater the forces holding the mold closed. This type of mold is therefore more commonly referred to as a self-locking mold.
The angle α is more or less determined by the ratio of the surface area of the shells subjected to the internal pressure and the surface area of the segments subjected to the said pressure. It will be noted that this ratio corresponds to the ratio of the diameter J of the mold at the interconnect point and the width L of the mold between the upper interconnect point and the lower interconnect point.
The forces involved are relatively high and the components are dimensioned in order to allow the smallest possible clearance, and so this may result in a mold the components of which are massive and, as a consequence, more expensive.